Smaller sensor, made by Japanese suppliers used to consumer electronics, instead of Blad's military sensors.

Much, much bigger quantities. Nikon's going to sell as many of these things in a month or two as Pentax, Blad, P1/Mamiya, and Leica, combined, sell medium format bodies in a year.

But, as I outlined elsewhere in this thread, Pentax does have advantages over Blad: they buy more sensors, they buy a hundred times more processors and displays, they can "recycle" DSLR software, and they don't farm out the bodies to Fuji, like Blad does.